mruby-regexp: fix MatchData#[] for a negative index and an unknown name - #7000

MatchData#[] resolves its argument in two branches, and both answer nil for
an argument CRuby handles differently: a negative index is rejected outright,
and a name that resolves to no group is treated as a failed match.

md = /(a)(b)/.match("ab")
md[-1]                       # CRuby: "b", mruby: nil
md[-2]                       # CRuby: "a", mruby: nil

/(?<x>a)/.match("a")[:zz]    # CRuby: IndexError, mruby: nil
/(?<x>a)/.match("a")["zz"]   # CRuby: IndexError, mruby: nil

Out-of-range positive indices already answer nil correctly and are unchanged.

A negative index

matchdata_aref() tested idx < 0 at the point where it reads the capture
table and gave up there, so counting back from the last group never worked.
Adding the group count in the numeric branch is the whole fix; the name branch
reaches the same label with a group number that is already non-negative, so
only the upper bound has to stay.

The lower bound follows CRuby's rb_reg_nth_match(), which discards the
adjusted index unless it is positive rather than merely non-negative:

    else {
        nth += RMATCH_REGS(match)->num_regs;
        if (nth <= 0) return Qnil;
    }

Two consequences are worth stating, because both look like bugs until you read
that:

md[-3]   # nil, not "ab": a negative index never reaches group 0
md[-4]   # nil, not IndexError: out of range downwards is nil like upwards

Both are pinned in the tests.

An unknown group name

A String or Symbol that names no capture group answered nil, which is
indistinguishable from a group that matched nothing, so a typo in a named
capture surfaced somewhere far from the mistake. CRuby raises IndexError, and
it does so even when the pattern declares no named group at all:

/(a)/.match("a")[:zz]   # CRuby: IndexError (undefined group name reference: zz)

The message is formatted with %l rather than %v. %v runs the value
through mrb_obj_as_string(), which dispatches to_s, so a redefined
Symbol#to_s could choose the text of an argument check. %l copies the bytes
directly, which also keeps matchdata_aref() free of any call back into the
VM.

Deliberately not in this change

• The md[start, length] and md[range] forms, which CRuby also accepts.
  [] is defined here with MRB_ARGS_REQ(1), and adding the extra forms is a
  larger piece of work.
• The TypeError message for an argument that is neither an index nor a name.
  md[nil] says nil cannot be converted to Integer where CRuby says
  no implicit conversion from nil to integer; that phrasing comes from
  mrb_as_int() and is mruby-wide, so it is not this gem's to change.
• MatchData#begin and #end, which take their argument as "i" and so
  reject a group name outright, and answer nil where CRuby raises. They are
  the same class of bug in the neighbouring methods, but their rules differ
  (they raise for every index they cannot use, including a negative one), so
  they are better handled on their own.

One more heads-up for whoever reviews this: the name lookup a few lines above
compares lengths after truncating the requested length to uint16_t while the
memcmp() next to it uses the untruncated length, which reads out of bounds
for a name longer than 65535 bytes. That is a memory-safety bug rather than a
compatibility one, it is reachable from plain Ruby, and it is not touched here.
I will send it as a separate one-line change so it can be reviewed and
backported on its own.

Testing

Two assert blocks in mrbgems/mruby-regexp/test/regexp.rb, one per branch,
covering the CRuby rules above including the two counter-intuitive ones. Every
row in this description was checked against CRuby 4.0.6 rather than assumed.

rake test is green on a default host build:

bintest is 105 OK, 0 KO. No existing test changed.

The two commits are independent and each passes the suite on its own, so
either can be dropped if only one of the two behaviours is wanted.

Summary by CodeRabbit

• Bug Fixes

  • Improved MatchData#[] behavior for negative capture indices.
  • Prevented out-of-range negative indices from incorrectly selecting the full match.
  • Undefined named capture references now raise IndexError instead of returning nil.

• Tests

  • Added coverage for negative indices and missing named captures.